1. Field of the Invention
The present invention relates to a material for organic electroluminescence device and an organic electroluminescence device (hereinafter also referred to as “device” or “organic EL device”). The invention relates to a technology capable of reducing a rate of occurrence of short-circuit device in the manufacture and enhancing manufacturing yields and a technology capable of enhancing storage stability of the device.
2. Description of the Related Art
In recent years, in view of the fact that light emission with a high brightness is obtainable through low-voltage driving, an organic electroluminescence device has been actively researched and developed. In general, the organic electroluminescence device is constituted of an organic layer including a light emitting layer and a pair of electrodes interposing this light emitting layer therebetween, and energy of an exciton generated through recombination of an electron injected from a cathode and a hole injected from an anode is utilized for the light emission.
In the organic electroluminescence device, an improvement of efficiency of the device is being advanced by using a phosphorescent material. As the phosphorescent material, there is known an iridium complex, a platinum complex or the like, which is capable of undergoing blue, green or red light emission. For example, US-A-2007/0190359 and US-A-2008/0297033 disclose an iridium complex of a ligand having a condensed ring structure.
The organic electroluminescence device is required to have high durability such that it endures long-term light emission. However, it is well known that penetration of moisture into the device generates a dark spot, thereby lowering the durability. It may be considered that this is caused due to the fact that a water molecule accelerates chemical decomposition of a material, or accelerates separation between an organic layer and an electrode interface.
In response to this, there have been made various countermeasures for the purpose of preventing a lowering of durability to be caused due to the penetration of moisture; and there have been disclosed a reduction of the water content in a thin film due to dehydration after film formation (see, for example, WO 01/058221); protection from the penetration of moisture by a sealing structure (see, for example, JP-A-2007-87620, JP-A-2006-294534 and JP-A-2006-278067); utilization of a moisture absorbing material or a moisture capturing material (see, for example, JP-A-2006-66366 and JP-A-2006-210095); and the like.
Also, it is disclosed that a heat treatment is carried out in vacuo during the film formation, thereby removing moisture (see, for example, JP-A-2003-313654).
Also, it is disclosed that in the case where a layer is formed by a wet film formation method, moisture contained in a composition for organic electroluminescence device is decreased as far as possible such that the moisture does not remain in the film after drying, thereby suppressing a lowering of characteristics of the device (see, for example, JP-A-2009-102656).
On the other hand, it is well known that inclusion of fine dusts into a device causes an electrical short circuit of the device, resulting in a lowering of manufacturing yields. As to a method of preventing the short circuit to be caused due to the inclusion of fine dusts, it is proposed that a flattened layer is provided between an anode layer and an organic layer (see, for example, JP-A-11-224781). However, according to this method, occurrence of a defective of the device to be caused due to the fine dusts in the organic layer cannot be avoided.
Also, it is proposed that a relative humidity at the time of film formation by a wet film formation method is regulated to 0.01 ppm or more, thereby suppressing the generation of static electricity during the film formation (see, for example, JP-A-2009-146691).
Also, it is proposed that a relative humidity in a pre-treatment step of vapor deposition is regulated to 0.01 ppm or more, thereby making it easy to control the circumstances constant and making it possible to stably manufacture a device (see, for example, JP-A-2008-192433).